Low temperature plasma incinerator and method of stabilizing impedance therein

ABSTRACT

Rapid incineration of organic materials in a low temperature plasma incinerator for generation into a plasma by an applied high frequency electric field supplied with oxygen gas and maintained at a high vacuum by a vacuum pump is promoted by introducing a secondary gas between the incinerator and the vacuum pump at a rate sufficient to maintain a generally constant vacuum in the incinerator and thus avoid deviations in the electric impedance balance of the system.

"United States Patent- 191 s U t 111 3,818,845

Nakane et al. I [4511 June 25,- 1974 [54] LOW TEMPERATURE PLASMA3,173,388 3/1965 Menrath et a] 110/8 E [NCINERATOR AND METHQD 03,357,376 12/1967 Miller 110/8 E Marr, .11. et al l R [75] Inventors:Hisashi Nakane, Kawasaki; Akira Uehara, Yokohama, both of Japan Prima'yExaminelr Meyer Peru Assistant Examiner-Ronald C. Capossela Assignee!Tokyo Ohka Kogyo Attorney, Agent, or Firm-William J. DanielKanagawa-ken, Japan [22] Filed: Dec. 20, 1971 5 [2]] Appl. No.: 209,8797] ABSTRACT Rapid incineration of organic materials in a low tem- I 30 FA perature plasma incinerator for generation into a 1 Drug pphcauon nomyData plasma by an applied high frequency electric field sup- DCC. 21,I970 Japan; yg g and maintained at a uumby a vacuum pump is promoted byintroducing a b51050? Secondary gas between the incinerator and the 9 l8 E uum pump at a rate sufficient to maintain a generally 0 careconstant vacuum in the incinerator and thus avoid deviations in theelectric impedance balance of the sys [56] References Cited tem UNITEDSTATES PATENTS 3,027,445 3/1962 Johnson 110/8 E 6 Claims, 1 DrawingFigure I FROM Pfi/MARY GAS SOURCE FROM szco/vomr 64.5 SOURCE H- T0 HIGHWCUUM SOURCE BACKGROUND OF THE INVENTION 1. Field of the Invention Thisinvention relates to a method for stabilizing impedance in a plasmalow-temperature incinerator.

2. Summary of the Prior Art In a low-temperature plasma incinerator,organic materials which are otherwise difficult to incinerate can bereadily incinerated into ashes within a plasma at a low temperature ofabout 100 to 200 C. Consequently, this type of oven is widely used inquantitative analysis for the pre-treatment of metals in an organicsubstance or for the incineration of photo-resists used for themanufacture of semi-conductors, et cetera.

The operation of the usual plasma low-temperature incinerator may besummarized as follows. A cylindrical reaction tube of quartz or glass isevacuated to a pressure less than about Torrs with a vacuum pump and ahigh-frequency high voltage is applied to electrode plates disposed onthe outside of the reaction tube by a high-frequency oscillatorconnected to the electrode plates, whereby a plasma of the gas remainingin the reaction tube is generated. If oxygen gas is introduced undersuch conditions into the reaction tube containing organic material, anoxygen plasma is produced, the material is incinerated to ashes by theaction of this oxygen plasma.

In such a system, theincineration rate is affected by various factorssuch as, for example, the level of voltage applied, the kind of gasintroduced (such as oxygen, nitrogen, hydrogen and the like), the rateof introduction of such gas, and the nature of its flow (either laminarflow or turbulent flow). The most important factor, however, is thematching of the impedance at the, electrical output of the oscillatorwith the impedance at the incineration reaction tube. The effect of suchmatching of impedance is great, particularly in the case of an apparatusof large processing capacity.

One of the causes of variation of such impedance is a change of thecapacitance on the output side relative to a prescribed capacitance onthe reactiontube side. Thus, when such changes occur, it becomesnecessary to adjust the grid voltage of the output tube and thisnecessarily results in a shift in impedance. However, such a shift isusually small and can be easily corrected by a variable condenser.

Another cause of a variation in impedance is a change in the flow rateof the gas introduced. It often occurs that the incineration operationis carried out by changing the flow rate of the gas introduced whilekeeping the power output constant. For instance, where the amount of thematerial to be treated is large, the gas may be initially introduced ina great quantity and gradually reduced in amount as theprocess proceeds.Also, when treating a wafer, the gas flow rate is adjusted according tothe thickness of the photo-resist layer thereupon. In general, impedancedecreases in proportion to increases in the level of vacuum, resultingin increased consumption of electric power. On the other hand, if thelevel of vacuum is lowered, the impedance increases and the powerconsumption is reduced. There exists, however, a level of vacuum belowwhich it may become impossible to generate any plasma. 7 I

It is known that in operating a plasma incinerator, a significantvariation in impedance may be produced when a small amount of gas isintroduced into a reaction tube held at a high vacuum so that the levelof vacuum in the tube is reduced, i.e., the pressure in the tubeincreasesQTherefore, impedance matching usually is accomplished by amanual adjustment of the condenser in accordance with the gas flow rate.However. if this technique of impedance matching is used for a highvacuum treatment, there can arise a situation where the plasma cannot beretained in the reaction tube but is released from the inlet or outlet,resulting in a condition similar tothat of a neon tube. If such acondition occurs, it is no longer possible to make correction by avariable condenser. Thus, a simple method for preventing variations inimpedance caused by change of the gas flow rate in a plasmalow-temperature incinerator has long been sought.

SUMMARY OF THE INVENTION GENERAL DESCRIPTION OF THE INVENTION Accordingto the present invention, there is provided a method and apparatus forpreventing variation in the impedance of the system in questioncharacterized in that a suitable flow of a secondary gas is introducedthrough a secondary gas inlet provided between the exhaust port of thereaction tube and the vacuum pump, in accordance with any increase ordecrease of the rate of primary gas flow into the reaction tube, whilethe interior of the reaction tube is kept in a high vacuum condition byuse of a vacuum pump having a large dis charge capacity, therebymaintaining constant the level of vacuum in the reaction tube bymaintaining the total flow of the two gases through the systemsubstantially constant.

According to the method of the present inventiomit is possible tosubstantially avoid any variation in impedance even if the primary gasflow rate to the reaction tube is varied from zero to the maximum levelfor practical use.

In practicing the concept of the present invention, no particulardifficulty is encountered. It is only required to provide a secondarygas inlet between the exhaust port of an incineration reaction tube anda vacuum pump in a conventional plasma low-temperature incinerator, andintroduce a secondary gas, such as, for example, air, through such.secondary gas inlet into the system. During operation of the incineratorin this manner, if the flow rate of the primary gas fed to the gas inletof the incineration reaction tube is reduced, the flow rate of thesecondary gas from the secondary gas inlet is increased; while, if thegas flow rate to the incineration reaction tube is increased, the gasflow rate from the secondary gas inlet is decreased. In this way,variations in the primary gas flow are compensated or balanced by anopposite variation in the secondary gas flow and the pressure in theincineration reaction tube is maintained substantially constant toprevent any change in impedance.

The incinerator reaction tube may include a cover or tion. The featureof the invention has the further ad vance that even if the operatorinadvertently neglects to disconnect the vacuum pump to restore normalpressure after completion of the incineration operation, no back flow ofoil to the vacuum pump will result since air or other secondary gas isallowed to flow in gradually through the inlet.

In practicing the present invention, it is preferred to use a vacuumpump having a large discharge capacity and the intake of the secondarygas is controlled so that the amount of secondary gas is larger than theamount of primary gas fed into each incineration reaction tube. In orderto obtain an excellent plasma, the level of vacuum in each incinerationreaction tube should be less than Torrs. Below 8 Torrs, a perfectlystabilized plasma can be obtained. For the case of cylindricalincineration reaction tubes each having a diameter of 75 mm and a lengthof 400 mm, the flow rate of primary gas introduced thereinto is usually1 l/min per tube at most.

The primary gas will ordinarily be oxygen to effect incineration of theorganic materials but, where other gases are known for use for thispurpose in the art, they are equally suitable in the context of thisinvention. The

secondary gas is of less importance since, being introduced downstreamof the incineration Zone, it does not affect the reaction. Air isextremely convenient for this purpose as it can be merely drawn from theatmosphere but other gas, which do not deleteriously affect the system,could be substituted if desired.

DESCRIPTION OF PREFERRED EMBODIMENT A plasma low-temperature incineratorconstructed as shown in the drawing was used. Three incinerationreaction tubes 1, 1' and 1" (each 75 mm in diameter and 400 mm inlength) having individual gas inlets, 2, 2' and 2" and gas dischargepipes 3, 3 and 3" were connected together through a distributingmanifold 4 and a collecting manifold 5. A gas feed inlet port 6 of thedistributing manifold 4 was connected through a pipe 9 to an oxygen gasbomb (not shown), with a flowmeter 8 having a needle valve 7 arranged ata suitable location along pipe 9, so as to allow oxygen gas to flow intothe system as a primary gas. The exhaust port 10 of the collectingmanifold 5 is connected to a vacuum pump preferably having a dischargecapacity of about 150 l/min (now shown). Between the exhaust port 10 andthe vacuum pump is provided a secondary gas inlet 11 to which aflow-meter 13 having a needle valve 12 is connected and through which asecondary gas (for example air) 14 is introduced. Along thecircumference of each incineration reaction tube 1, l' and 1" areprovided pairs of opposed metal plates l5, l6, l5, l6 and 15", 16" toserve as electrodes, to which a 3.000- volt, 13.56-MI-Iz high-frequencyoscillator is connected.

With the mechanism assembled in this manner, each needle valve is closedand the vacuum pump is operated until the pressure in each incinerationreaction tube is reduced to 0.2 Torrs in about 3 minutes. Upon reachingthis stage, a high-frequency output power of about 300 W is applied tothe electrodes of each incineration reaction tube, whereupon generationof plasma is observed and the impedances at the electrical output of theoscillator and at the incineration reaction tubes are matched byadjusting a variable condenser in the oscillator, If the output isincreased to the level of 1,000 W under this condition, plasma spreadsout to gas inlet and outlet ports of each incineration reaction tube (asin the case of a neon tube) and, when this occurs, it is no longerpossible for the variable condenser to control the plasma so as toretain it inside the reaction tubes. Therefore, the output power isusually reduced to the level of 500 W in operation, but this is noteconomical. When under such low output condition oxygen gas isintroduced into each incineration reaction tube at a flow rate of 200 to300 ml/min, the pressure in each tube slightly varies within the rangeof 0.2 to 0.5 Torrs, causing decrease of the output to 400 W. When theflow rate is increased to 500 ml/min, the pressure is increased to 1Torr, causing a further decline in the output to 300 W. The matchingimpedance condition could be restored by operating the variablecondenser manually in accordance with the change of the flow rate tothereby obtain the approximately initial output power. In this case, theamount of control by the variable condenser is proportional to'the flowrate.

Then, in'accordance with the invention, before generating plasma, airwas introduced from the secondary gas inlet, the flow rate of aircontrolled by the needle valve so that the flow rate is graduallyincreased from zero and the shift of the impedance matching point, thecondition of plasma in the tubes, and the output were observed. Theresults are shown in the Table below.

Effect on Secondary Gas Introduction on Conditions of Incineration Gasflow rate into Air flow incin. re- Impedance Incinerarate I/ PressureTemp I act. tube Output matching tion time min) (Torr) Condition ofplasma (C) (ml/min) (Watts) point (Min.)

0 0.2 Spread to outside of tubes 0 1700 15 I 60 0 1600 2 60 0 1500Shifted 3 60 0 1400 5 4 1 0-300 1300 5 I 100 0-300 1300 Slightly 3 6 2100 0-300 1100 Shifted 7 3 Plasma at outlet vanishes & is fixed in tubesI60 0-300 1000 1.5-2 8 3 Plasma at inlet also vanishes & is fixed intubes... 0-300 1000 1.5-2 9 3 160 0-300 1000 10 3 160 0-300 1000 ll 3160 0-300 1000 Q 12 3 160 0-300 1000 Shift 13 3. 160 0-300 1000 14 "3.160 0-300 1000 15 4 I60 0-300 I000 Note 1: The air flow rate is thevalue for three incineration reaction tubes combined.

Note 2: The gas flow rate into incineration reaction tube is the valuefor a single incineration reaction tube.

Note 3: The output is the value for the three incineration reactiontubes combined.

As apparent from the results shown in the above table, an idealstabilized gas balance effect is produced when the air flow rate fromthe secondary gas inlet is made higher than the gas flow into theincineration reaction tubes, particularly when it is maintained at. 8 to14 l/min.

Then, a piece of wafer made of OMR81 (photo-resist manufactured by TokyoApplied Chemicals Co.) having a film thickness of l p was placed in eachof the three incineration reaction tubes and plasma was generated underthe same conditionsas described above for determining the time requiredfor incineration. The results are also shown in the above table.

. It is evident from these results that the incineration rate is highwhere good impedance matching exists.

What is claimed is:

1. in a method of incinerating organic material in a low temperatureplasma incinerator wherein a plasmaforming incinerating gas is fed intoan enclosed incinerating zone containing said material and is convertedinto a low temperature plasma by means of a high frequency electricfield passing through said zone while the zone is maintained under ahigh vacuum by communication with a vacuum source, and the amount ofsuch gas is varied during the course of incineration, the improvement ofintroducing downstream of said zone a'secondary gas in an amountsufficient to maintain in said incinerator zone a substantially constantlevel of vacuum during the incineration.

2. The methodof claim 1 wherein said incineration temperature is betweenabout lOD200 C.

3. The method of claim 1 wherein said high vacuum is not higher thanabout 10 Torr.

4. The method of claim 1 wherein the amount of the secondary gasintroduced downstream of said incinerating zone exceeds that of theoxygen supplied to the incinerating zone.

5. In a low temperature plasma incinerator for organic materials whichincludes at least one ceramic incinerator reaction tube for containingthe material to be incinerated and having inlet and outlet ports, meansfor supplying gaseous oxygen to said inlet port, conduit means forconnecting said outlet port to a high vacuum source, and meansforcreating a high frequency electric field within each such tube forconverting said oxygen to low temperature plasma and comprising at leastone pair of electrode plates arranged on opposite exterior sides of saidtube, each such pair of plates being in opposed spaced relation withsaid tube therebetween, and means for connecting said plates to a highhe quency high voltage electrical source, the improvement of a secondaryinlet port in said conduit between said reaction tube outlet port andsaid high vacuum source connected to a source of secondary gas, andadjustable valve means for controlling the amount of secondary gasadmitted through said secondary inlet port, whereby variations in thequantity of oxygen supplied to said tube can becompensated by admissionof the secondary gas to maintain the tube under substantially constantvacuum.

ments of said tube.

1. In a method of incinerating organic material in a low temperatureplasma incinerator wherein a plasma-forming incinerating gas is fed intoan enclosed incinerating zone containing said material and is convertedinto a low temperature plasma by means of a high frequency electricfield passing through said zone while the zone is maintained under ahigh vacuum by communication with a vacuum source, and the amount ofsuch gas is varied during the course of incineration, the improvement ofintroducing downstream of said zone a secondary gas in an amountsufficient to maintain in said incinerator zone a substantially constantlevel of vacuum during the incineration.
 2. The method of claim 1wherein said incineration temperature is between about 100*-200* C. 3.The method of claim 1 wherein said high vacuum is not higher than about10 Torr.
 4. The method of claim 1 wherein the amount of the secondarygas introduced downstream of said incinerating zone exceeds that of theoxygen supplied to the incinerating zone.
 5. In a low temperature plasmaincinerator for organic materials which includes at least one ceramicincinerator reaction tube for containing the material to be incineratedand having inlet and outlet ports, means for supplying gaseous oxygen tosaid inlet port, conduit means for connecting said outlet port to a highvacuum source, and means for creating a high frequency electric fielDwithin each such tube for converting said oxygen to low temperatureplasma and comprising at least one pair of electrode plates arranged onopposite exterior sides of said tube, each such pair of plates being inopposed spaced relation with said tube therebetween, and means forconnecting said plates to a high frequency high voltage electricalsource, the improvement of a secondary inlet port in said conduitbetween said reaction tube outlet port and said high vacuum sourceconnected to a source of secondary gas, and adjustable valve means forcontrolling the amount of secondary gas admitted through said secondaryinlet port, whereby variations in the quantity of oxygen supplied tosaid tube can be compensated by admission of the secondary gas tomaintain the tube under substantially constant vacuum.
 6. Theincinerator of claim 5 wherein each such tube is generally tubularshaped and said electrode plates are arcuately curved around oppositeperipheral segments of said tube.